Yes, you just need to win a certain number of races in each class of vehicle the number seems to be 30 to 40 races after reaching about 50% of completion (regardless of whether you've completed the $1mil mission w/Karol).

No you cant but the coler says difficulty (greeen:easy yello:medium orang:hard red:hardest) red gives most money actualy if you get ninja and you race a lambo you get 7,000 if you win the faster car the more cash

You don't. You have to go to a garage and buy the scissor doors. GLITCH: To drive with doors open, go to the 'Doors' menu, when your doors open, press O (or B, depends on console), then press X (or A) immediately after.